Patients with the "lupus anticoagulant", a clotting abnormality associated with circulating antibodies to phospholipids, are at increased risk for both venous and arterial thromboses. Pregnant women with such antibodies are at high risk for fetal loss, presumably due to thrombosis at the level of the spiral arterioles or within the maternal floor of the placenta. Affected women with repeated pregnancy loss have been treated prophyllactically in subsequent pregnancies with immunosuppression and anticoagulation. Pregnancies at risk permit comparison of the effectiveness of prophyllactic immunosuppression or anticoagulation in the prevention of thromboses over a limited period of time with a specific treatment end point. We plan a prospective, randomized collaborative trial of immunosuppression versus anticoagulation for pregnant patients at risk for this complication and will compare both fetal outcome and maternal safety of these two regimens. A specific immunoassay will be used for measurement of maternal serum levels of antibody to tailor immunosuppressive therapies for the maintenance of normal serum antibody levels.